In these studies we are attempting to define the immunologic function of cellular subsets bearing the Leu 8 (LAM-1) molecule. During this period we directed our attention to the function of the Leu 8 molecule on various cell types. In studies with CD4+ T cells we showed that cross-linking Leu 8 on the cell surface with solid-phase anti-Leu 8 greatly augments proliferation of CD4+ T cells induced by anti-CD3, while having no independent effect on CD4+ T cell proliferation alone. These studies suggest that the Leu 8 molecule interacts with the CD3 signal transduction complex. In other studies involving CD4+ T cells we showed that Leu 8 expression is down-regulated at both the surface protein level and the mRNA level by cell activation, but this down-regulation is rapidly reversible during culture. Loss of Leu 8 following cell activation, however, does not explain the lack of Leu 8 expression in T cells of the lamina propria since the latter do not regain Leu 8 expression during culture and do not express IL-2 mRNA. Nevertheless, these cells were probably derived from Leu 8+ cells because the Leu 8 gene of Leu 8- T cells is partially demethylated, i.e., it was once transcriptionally active. The process causing permanent loss of Leu 8 expression in lamina propria T cells remains unknown. Turning attention to the Leu 8 molecule on neutrophils we showed that most Leu 8 molecules on the surface neutrophils have a higher molecular weight than do Leu 8 molecules on lymphocytes and, concomitantly, Leu 8 mRNA derived from neutrophils is predominantly the higher molecular weight species of the two species present. This suggests that most Leu 8 associated with neutrophils is the membrane anchored form other than the phosphatidyl-inositol (PI)-linked form and, indeed, very little Leu 8 is cleared from neutrophils with PI-linked proteins (those with paroxysmal nocturnal hemoglobinuria) have a more or less normal complement of Leu 8. Finally, in studies of the Leu 8 molecule on B cells we showed that cross-linking of the Leu 8 molecule with solid phase anti-Leu 8 suppressed SAC-induced Ig secretion of Leu 8 B cells almost completely. This striking effect was obtained without any effect on B cell proliferation, IL-2R , or c-myc mRNA expression. These studies indicate that the indirect down-regulation of B cell differentiation mediated by cross-linking of Leu 8 on T cells is complemented by a direct negative effect by cross-linking of Leu 8 on the B cell itself.